Thin film transistors are used as switching elements or integrated elements of peripheral driving circuits in various display devices. Presently, the widely used thin film transistors mainly include amorphous silicon thin film transistors and polycrystalline silicon thin film transistors. However, since the amorphous silicon thin film transistors have low mobility and easy performance degradation, the applications in the pixel driving of an OLED and in the integration of peripheral driving circuits of a LCD and OLED are limited. Moreover, manufacturing the polycrystalline silicon thin film transistors needs a high temperature, the manufacturing cost is high, and the performance uniformity of the polycrystalline silicon thin film transistors is poor. Thus, the polycrystalline silicon thin film transistors are not suitable to the large-size panel displays. Therefore, for developments of the large-size panel displays, it is needed to develop more advance thin film transistors. Currently, the new developing thin film transistors mainly include metal oxide semiconductor thin film transistors represented by the zinc oxide semiconductor thin film transistors, microcrystalline thin film transistors and organic semiconductor thin film transistors.
The zinc oxide based and indium oxide based thin film transistors have low manufacturing process temperatures, low manufacturing cost, high carrier mobility, and uniform and stable performance. That is, the zinc oxide based and indium oxide based thin film transistors have the advantages both of the amorphous silicon thin film transistors and of the polycrystalline silicon thin film transistors, and are large-size microelectronic devices having a good prospect. However, there is a disadvantage in the method for manufacturing the zinc oxide based thin film transistors. Specifically, the resulting zinc oxide based thin film transistors have non-self-aligned structures, resulting in large parasitic elements existing in the transistors and uncontrollable characteristics discreteness. The parasitic capacitances do great damage to uncontrollable characteristics discreteness. The parasitic capacitances do great damage to performances of a pixel driving unit and a peripheral driving circuit. Existing means for eliminating the impact of the parasitic capacitance tends to increase complications of the transistor's structure and manufacturing process. There is another problem in the zinc oxide based thin film transistors. That is, the zinc oxide based thin film transistors have such disadvantages that the formed semiconductor channel layer tends to have a very high carrier concentration, so that the threshold voltage of the transistors is very low and even negative (for the n-typed transistors). That is to say, when the gate is in the state of zero bias, the transistor cannot be turned off sufficiently. If the channel layer is fabricated into a high-resistance layer having a low concentration, the parasitic resistance of source and drain regions will be increased accordingly. Therefore, there is needed to add a metal layer having low-resistance, resulting in a more complicated process.